Since you have the ability to dump the yeast out of the bottom at will, you are getting the beer off of the yeast without transferring it. Every few days I'll go in and dump a cup of yeast sludge. The processes are different, but the effect is the same. However, with a conical, you can get the yeast away from the beer whenever you want and with a LOT less hassle.

The 15 gal version is ideal for 10-13 gal batches. But you can ferment smaller batches in them. If you do, you may not notice alot of airlock activity--the CO2 bubbles need to saturate the beer first, then fill the headspace, before they ever start coming out of the airlock. So it may appear that nothing's happening, when in fact it is. If your boiling kettle isn't large enough to handle batches that big, I'd stick with the standard 5 gal batches. Incidentally, do you perform partial boils or full boils, and how big is your kettle?

Lastly, when determining your batch size, you'll need to account for the loss below the racking port and for all the trub dumps you plan on making. If I recall correctly, my loss below the racking port is 3/4 gallon. And if I do four trub dumps at a cup each, there's another quart. So my total loss is one gallon. If you want to end up with 4.5 gals in a keg, or bottles, then you'll needed to put 4.5 + 1 gals into the fermenter, or 5.5 gals at least.

I do a partial boil and my kettle I use now is about 3-4 gals. I do have larger kettle that needs to be cleaned out and can do about 5 gals I think. But I was thinking of getting a bigger kettle so I could do a full boil and I boil on a propane burner. So my best bet is to get a setup for around 5-6 gallons and use my bucket and carboy for additional batches. Or if I can talk my wife into buying me a couple of them then I would be in business! Worth a try!
Thanks again! Info here always helps.

for doing a full boil for a 5 gallon batch, you should have an 8 gallon pot. Because if you want 5.5 gallons to go to the primary, then you will be boiling close to 6 1/2 or 7 gallons depending on how vigorous your boil is. And you need a little space above that for the foam head right when it starts to boil (dont want a boilover) So I would recommend getting a 32 qt pot ( or bigger if you think you might go to 10 gal batches at some time. Pots are expensive so buy with the idea that you might go BIG, cuz then you will already have what you need. You can always boil a smaller amount in a big pot!!!